1. Field of the Invention
In general, the present invention relates to a method of fabrication of photomasks, sometimes referred to as reticles. In particular, the photomasks are fabricated using a direct write process which employs exposure of a chemically amplified photoresist to laser-produced Deep Ultra Violet (DUV) radiation or to electron beam radiation.
2. Brief Description of the Background Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of semiconductor device structures. The miniaturized electronic device structure patterns are typically created by transferring a pattern from a patterned masking layer overlying the semiconductor substrate rather than by direct write on the semiconductor substrate, because of the time economy which can be achieved by blanket processing through a patterned masking layer. With regard to semiconductor device processing, the patterned masking layer may be a patterned photoresist layer or may be a patterned xe2x80x9chardxe2x80x9d masking layer (typically an inorganic material or a high temperature organic material) which resides on the surface of the semiconductor device structure to be patterned. The patterned masking layer is typically created using another mask which is frequently referred to as a photomask or reticle. A reticle is typically a thin layer of a metal-containing material (such as a chrome-containing, molybdenum-containing, or tungsten-containing material, for example) deposited on a glass or quartz plate. The reticle is patterned to contain a xe2x80x9chard copyxe2x80x9d of the individual device structure pattern to be recreated on the masking layer overlying a semiconductor structure.
A reticle may be created by a number of different techniques, depending on the method of writing the pattern on the reticle. Due to the dimensional requirements of today""s semiconductor structures, the writing method is generally with a laser or e-beam. A typical process for forming a reticle may include: providing a glass or quartz plate, depositing a chrome-containing layer on the glass or quartz surface, depositing an antireflective coating (ARC) over the chrome-containing layer, applying a photoresist layer over the ARC layer, direct writing on the photoresist layer to form a desired pattern, developing the pattern in the photoresist layer, etching the pattern into the chrome layer, and removing the residual photoresist layer. When the area of the photoresist layer contacted by the writing radiation becomes easier to remove during development, the photoresist is referred to as a positive-working photoresist. When the area of the photoresist layer contacted by the writing radiation becomes more difficult to remove during development, the photoresist is referred to as a negative-working photoresist. Advanced reticle manufacturing materials frequently include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example.
As previously mentioned, the reticle or photomask is used to transfer a pattern to an underlying photoresist, where the reticle is exposed to blanket radiation which passes through open areas of the reticle onto the surface of the photoresist. The photoresist is then developed and used to transfer the pattern to an underlying semiconductor device structure. Due to present day pattern dimensional requirements, which are commonly less than 0.3 xcexcm, the photoresist is typically a chemically amplified photoresist. In the making of the reticle itself, a chemically amplified DUV photoresist has been used in combination with a laser-produced DUV radiation or a direct write electron beam writing tool. An example of a continuous wave laser-produced DUV direct write tool is available under the trade name ALTA(trademark) from ETEC Systems, Inc., Hillsboro, Oreg. An example of an electron beam direct writing tool is available under the trade name MEBES(trademark) from ETEC Systems, Inc., Hayward, Calif.
Preparation of a photomask/reticle is a complicated process involving a number of interrelated steps which affect the critical dimensions of a pattern produced in the reticle, including the uniformity of the pattern critical dimensions across the surface area of the reticle. By changing various steps in the reticle manufacturing process, the reproducibility of the manufacturing process itself may be altered, including the processing window. Processing window refers to the amount process conditions can be varied without having a detrimental outcome on the product produced. The larger the processing window, the greater change permitted in processing conditions without a detrimental affect on the product. Thus, a larger process window is desirable, as this generally results in a higher yield of in specification product produced.
One processing variable which has significantly reduced the processing window for photomask fabrication is the shelf life of a reticle substrate with the photoresist applied over its surface. As previously mentioned, the photoresist used for pattern imaging of 0.3 xcexcm or less feature sizes is typically a chemically amplified photoresist. The chemically amplified photoresist (CAR) is generally designed to produce an acid in the area irradiated by ultraviolet light, laser light, X rays or an electron beam. The irradiated area forms an image in the CAR which is subsequently developed into a pattern. The acid produced renders the irradiated portion of the CAR soluble in a basic developing solution. Many variations of chemically amplified resists are commercially available primarily for 257 nm, 248 nm, and 193 nm deep ultraviolet (DUV) light lithography application. Many of these CARs have been used in electron beam light lithography.
It is generally known that photoresists, and especially CAR, are sensitive to certain environmental contaminants, thus rendering their use for mask fabrication somewhat problematic, often requiring special handling. It has been found that CAR deteriorates in terms of lithographic performance as soon as one hour (or less) after its application over a substrate. To prevent this, applicants developed a protective coating for application over the CAR, as a means of extending the time a photomask substrate with CAR applied could be stored prior to exposure to the imaging/patterning radiation. However, subsequent to development of the protective coating, we discovered that reproducibility of patterning during the direct write imaging processing was not good. A direct write process for a photomask may take up to about 20 hours, and during the 20 hour time period, the photoresist was being affected in a manner so that the critical dimension of the patterned feature was becoming smaller as the direct writing on the photoresist progressed. The present invention solves the problem of how to maintain a uniform and reproducible pattern critical dimension in the CAR during the direct writing process for imaging a photomask.
In photomask making, the environmental sensitivity of a chemically amplified photoresist is eliminated, or at least substantially reduced, by overcoating the photoresist with a thin coating (topcoat) of a protective but transmissive material. It is particularly helpful if the topcoat material exhibits a refractive index and thickness which is matched to the refractive index and thickness of the photoresist. The representative equation is: t=xcex/4n, where t=thickness, xcex=wavelength of light passing through the topcoat or photoresist, and n=refractive index. For Deep UV direct write imaging, typically the chemically amplified photoresist thickness is in the range of about 5000 xc3x85, and the refractive index varies from about 1.79 to about 1.83. As a result, the thickness of the topcoat is typically ranges from about 650 xc3x85 to about 675 xc3x85, and the refractive index of the topcoat ranges from about 1.32 to about 1.45. In general, the refractive index of the topcoat≈the square root of the refractive index of the chemically amplified photoresist.
To provide improved stability during the long time period required for direct writing of a photomask pattern, typically in the range of about 20 hours, the topcoat is pH adjusted to be as neutral in pH as possible, depending on other process variable requirements. Although a topcoat pH of about 6.5 to about 7.5 may be used in some instances, a pH ranging from about 5 to about 8 may generally be used, and is often necessary when the topcoat is to be charge dissipating (such as for e-beam lithography, where the topcoat is rendered conductive). The conductivity of the topcoat may assist in maintaining the shelf life of the underlying photoresist in general, during storage in handling which may be for time periods up to weeks. In addition, the conductivity assists in providing stability of the photoresist during the direct write process. By adjusting the pH of the topcoat to fall within a range of about 5 to about 8, both the storage life of a photoresist-coated substrate is increased, and the stability during direct write imaging of the photoresist by either optical or e-beam radiation is significantly improved, providing substantial improvement in lithographic performance (in terms of critical dimension and integrity) of the photoresist.
There are instances where the direct writing upon the photoresist is conducted using a tool which is optical in nature. An example would be a direct write continuous wave laser which produces DUV radiation. In this instance, a more neutral topcoat having a pH in the range of about 6.5 to 7.5 may be used. For example, U.S. patent application, Ser. No. 09/848,859, filed May 3, 2001, titled: xe2x80x9cOrganic Bottom Antireflective Coating For High Performance Mask Making Using Optical Imagingxe2x80x9d, process which employs a new direct pattern writing tool which is a 244 nm or a 257 nm mask writing laser available from ETEC Systems Inc., Hillsboro, Oreg. Additional information about process variables in the 257 nm direct writing of photomask patterns is provided in U.S. application, Ser. No. 09/912,116, filed Jul. 23, 2001, titled: xe2x80x9cMethod Of Preparing Optically Imaged High Performance Photomasksxe2x80x9d, also assigned to the assignee of the present invention. Both of these applications are hereby incorporated by reference in the present application, in their entirety.
By application of a substantially neutral pH protective topcoat over a CAR, it is possible to prepare an unexposed photoresist-coated substrate (wafer or reticle) months before its actual exposure, and to maintain the substrate in a patterning (imaging) tool for long time periods, using the method of the present invention. This is in contrast with the use of an unprotected CAR which requires application of the photoresist to the substrate immediately prior to the pattern writing, and which does not provide as consistent a critical dimension during the entire photomask direct writing process.